1. Technical Field
The present invention provides a catalyst system for the polymerization of olefins that include the combination of a titanium catalyst with an electron donor compound of the formula: ##STR2## wherein R.sub.1 is an alkyl group of the formula --CH.sub.2 R' in which R' is hydrogen or an n-alkyl group; R.sub.2 and R.sub.3 are alkyl or aryl groups; and R.sub.4 is an alkyl group of the formula --CH.sub.2 R' in which R' is hydrogen or an n-alkyl group, R.sub.1 and R.sub.4 are the same or different.
2. Description of Prior Art
Catalyst systems for the polymerization of olefins are well known in the art. Typically, these systems include a Ziegler-Natta type polymerization catalyst; a co-catalyst, usually an organoaluminum compound; and an electron donor, usually an organosilicon compound. Examples of such catalyst systems are shown in the following U.S. Pat. Nos.: 4,107,413; 4,294,721; 4,439,540; 4,115,319; 4,220,554; 4,460,701; and 4,562,173; the disclosures of these patents are hereby incorporated by reference. These are just a few of the scores of issued patents relating to catalysts and catalyst systems designed primarily for the polymerization of propylene and ethylene.
A Ziegler-Natta type polymerization catalyst is basically a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium with a metal hydride and/or a metal alkyl that is typically an organoaluminum compound. The catalyst is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkylaluminum.
The development of polymerization catalysts has improved catalyst properties, particularly, the efficiencies of the catalysts, as expressed in kilograms of polymer product per gram of catalyst in two hours, have increased from the 1-3 range to the 10-12 range and beyond. Catalysts have been developed that not only have higher efficiencies but also retain their activity over a longer period of time, thus being able to produce more polymer product over the life of the catalyst. Any increase in the efficiency and life of the catalyst leads to lower costs due to lower catalyst consumption, and it also lowers the capital expenditures in building and operating a plant as the size of the reactors are lowered for specific plant capacities and the reactor residence time is lowered. A higher efficiency also leads to a cleaner polymer product thereby avoiding the need to wash or treat the product to remove catalyst ash residuals.
In addition to the improved catalysts, improved activation methods have also lead to increases in the catalyst efficiency. A process for pre-polymerizing the catalyst just prior to introducing the catalyst into the reaction zone is disclosed in U.S. Pat. No. 4,767,735 the disclosure of which is hereby incorporated by reference.
In addition to the development of new catalysts and new reaction processes, a discovery of a more appropriate co-catalyst or electron donor to go with the new generation of catalysts in forming a total catalyst system would be of great benefit to the polymerization art, especially if it leads to dramatic increases in the efficiency of the catalyst system and to improved quality control of the polymer product. In such a total catalyst system, a co-catalyst activates the catalyst and provides the initiation of a polymer chain. A co-catalyst that works well with supported Ziegler-Natta catalysts for the polymerization of olefins is an organoaluminum compound, most typically triethylaluminum (TEAI) or another trialkylaluminum, such as triisobutylaluminum. Examples of other useful organoaluminum compounds include an alkylaluminum dihalide and a dialkylaluminum halide.
An electron donor compound is used in the polymerization reaction to reduce the atactic form of the polymer thereby giving control of and increasing the production of isotactic polymers. Although a broad range of compounds are known generally as electron donors, a particular catalyst may have a specific compound or group of compounds with which it is especially compatible. Work disclosed in Makromol. Chem., Harkonen et al, vol. 192, p. 2857 (1991) and in "Catalytic Olefin Polymerization"(Keii, T., Soga, K., Eds., Elsevier:New York, 1990) Harkonen et al, p. 87, has shown that some effects of the donor can be influenced by its structure and suggests that a high performance alkoxysilane donor will contain at least two small alkoxy groups, preferably methoxy and one or two branched alkyl groups. Other work suggested that a good donor preferably has a dimethoxy structure with a bulky alkyl or cycloalkyl group containing at least one secondary or tertiary carbon, but that the second alkyl group should contain a primary carbon. One such group of electron donors are disclosed in U.S. Pat. No. 4,927,797.
The present invention comprises another such discovery of an appropriate type of electron donor which gives significant increases in catalyst efficiency as well as improved control of the isotactic index of the desired polymer product and other properties of the product would be highly advantageous. It has been surprisingly discovered that a specific group of silane compounds serving as electron donors in combination with a particular type of catalyst results in significant increases in catalyst efficiency over the previously known efficiencies for this particular type of catalyst as well as other known catalyst systems.